1. Field of the Invention
The present invention relates to permanent magnet type electrical rotating machines, and more particularly to permanent magnetic type electrical rotating machines in which the construction of the stators is improved in order to prevent reduction of the properties of the permanent magnets that are the rotors.
2. Description of the Related Art
FIG. 1 is a partial cross-section view, in a direction orthogonal to the axis, showing an example of a prior art permanent magnet type electrical rotating machine. FIG. 2 is an enlarged development view of part C of FIG. 1.
In FIG. 1 and FIG. 2, slots 3F are formed at equal intervals on the inner periphery of stator core 2H that is formed in a cylindrical shape by laying silicon steel punched plates one on another. Stator winding 4B is inserted into these slots 3F in two layers. Trapezoidal wedges 11, fabricated of magnetic material described later, are press-fitted into the inner periphery sides of slots 3F. Tooth-like parts 10B of the stator core are formed between slots 3F.
Wedges 11 are made of hardened material by mixing malleable iron powder with resin and a reinforcing agent to become high-resistance magnetic material with a relative magnetic permeability (.mu.) of about 10.about.100.
At the same time, permanent magnets 6 of arc-shaped cross-section are mounted in close contact with the outer periphery of rotor core 7 in the peripheral direction and bonded with an adhesive agent to form rotor 5. Magnets magnetized with S poles on the inner periphery side and N poles on the outer periphery side are mounted alternately with magnets magnetized with the reverse polarity, according to the number of poles.
Prevention of the peeling-off of permanent magnets 6 due to the centrifugal force generated by high-speed rotation is designed by press-fitting cylindrical retaining ring 8 on the outer peripheries of these permanent magnets 6. Specified gap 9 is formed between the outer periphery of retaining ring 8 and inner periphery of stator core 2H.
In a permanent magnet type electrical rotating machine constructed in this way, magnetic flux 13 emanating from the outer periphery side of permanent magnets 6 reaches the outer periphery side of stator core 2H from wedges 11 and tooth-like parts 10B between these wedges 11 after passing through retaining ring 8 and gap 9, as shown in FIG. 2. From this outer periphery side, the flux once more passes through a magnetic path via the neighbouring permanent magnet poles.
This permanent magnetic type electrical rotating machine is driven in rotation at high speed by increasing the frequency of the inverter power source that excites stator winding 4B.
FIG. 3 is a drawing corresponding to FIG. 2, and is an enlarged partial development illustration showing the magnetic flux distribution emanating from a permanent magnet for the case when non-magnetic wedges 12 are used in place of magnetic material wedges 11.
In FIG. 3, the point of difference from above-mentioned FIG. 2 is that the greater part of magnetic flux 13 emanating from permanent magnets 6 passes through tooth-like parts 10B of stator core 2H while hardly any passes through non-magnetic wedges 12.
That is to say, hardly any flux passes through the inner periphery side of stator winding 4B, but is concentrated in the trapezoidal parts of the inner periphery sides of tooth-like parts 10.
Consequently, the peaks of the sine wave of the magnetic flux passing between permanent magnets 6 and the stator oscillate as shown in FIG. 4(a), and therefore the rotor torque oscillates.
On the other hand, with a permanent magnet type electrical rotating machine that incorporates magnetic material wedges 11 shown in FIG. 2, eddy currents flow in wedges 11 due to the magnetic flux passing through wedges 11. Thus, not only does the temperature of wedges 11 rise, but since, as mentioned above wedges 11 are made of hardened material , wedges 11 are very brittle, extreme care is required in their manufacture and assembly processes.
Moreover, eddy currents also flow in retaining ring 8 due to the magnetic flux passing through retaining ring 8. Since permanent magnets 6 are heated when the temperature of retaining ring 8 rises, the magnetic properties (coercive force) of permanent magnets 6 reduce.